HST image of Hickson compact group 87, HCG 87

    Features:

    1. The spiral galaxies in the Hickson Compact Group show the obvious complex mix of colors of star formation: blue (from hot young OB stars), pink (from the red atomic hydrogen line Hα), and brown (from obscuration by interstellar dust).

    2. The Hickson Compact Group galaxy to the right is HGC 87b which is a Hubble sequence type Lenticular S0 galaxy. However, it looks like an elliptical galaxy to the eye. It shows NO obvious star formation colors just like typical elliptical galaxies.

    3. Elliptical galaxies are usually quenched galaxies: i.e., galaxies where star formation is largely turned off.

    4. Why do galaxies get quenched and stay quenched? It's a complex and NOT yet fully resolved story. But currently there are two main processes that are considered. First, smallish galaxies suffer galaxy ram-pressure stripping of their gas in galaxy clusters by intergalactic gas, and so have no gas to form stars. Second, large galaxies above the golden mass ≅ 10**12 M_☉ (including dark matter which has about 10 times more mass than the baryonic matter) have strong AGN feedback from their central supermassive black holes that turns off star formation. The idea is that outflows of hot gas from near the accretion disk of a central supermassive black hole pushes out to the intergalactic medium (IGM) or heats up colder gas in the galaxy that could form stars if it were not pushed out or too hot to collapse from loss of pressure. There is some inflow of colder gas into the galaxy from the IGM, but it is insufficient for star formation.

    5. The golden mass ≅ 10**12 M_☉ is NOT set by a law of nature. It is just an approximate value determined by the interaction of the components of the evolving observable universe.

    6. How do large elliptical galaxies form? Galaxy mergers randomize the orbits of the stars of the largish merging galaxies creating an new elliptical galaxy. The merging galaxies could be of any galaxy types including elliptical galaxies.

      If the merging galaxies both have star formation, the galaxy merger will likely cause a major starburst region (i.e, causes the merging galaxies to become a starburst galaxy with a very high star formation rate (SFR)). The colliding interstellar medium (ISM) is the cause of the starburst. However, if the golden mass ≅ 10**12 M_☉ is exceeded, the merged galaxy will quench on a time scale of 2 Gyr???. Galaxy quenching of this sort starts in the inner region and progresses outward: this is inside-out galaxy quenching. If the merged galaxy does NOT quench, then its ISM can relax to a galactic disk and the merged galaxy could evolve into spiral galaxy.

    Local file:     local link: galaxy_hcg_87_2c.html.
    File: Galaxies file: galaxy_hcg_87_2c.html.